As methods for producing a color filter which is used for a liquid crystal display element and a solid-state camera element, a dyeing method, a printing method, an electrodeposition method and a pigment dispersing method are well-known.
A dyeing method is a method of producing a color filter by dyeing a base material to be dyed comprising a natural resin such as gelatin, glue, casein, etc., or a synthetic resin such as amine-modified polyvinyl alcohol, etc. with a dye such as an acid dye.
As dyes are used in a dyeing method, there are various problems such as light fastness, heat resistance, moisture resistance, etc. In addition, it is difficult to control dyeing and fixing properties uniform in a big image plane and uneven coloring is liable to occur. Further, a dyeing protecting layer is necessary in dyeing, therefore, processes are complicated.
An electrodeposition method is a method of producing a color filter by previously forming a transparent electrode of a prescribed pattern, ionizing a resin containing a pigment dissolved or dispersed in a solvent, and applying voltage to form a colored image in a pattern.
In an electrodeposition method, a photolitho process comprising a stage of forming a film of a transparent electrode for color filter formation and a stage of etching is necessary in addition to the formation of a transparent electrode for display. If short circuit is caused at that time, it results in a line defect leading to yield reduction. An electrodeposition method is theoretically difficult to be applied to arrangement other than stripe, e.g., a mosaic arrangement. Further, there is such a problem as the control of a transparent electrode is difficult.
A printing method is a simply easy method of producing a color filter by printing such as offset printing using inks comprising pigments dispersed in a thermosetting resin or an ultraviolet-curing resin, but as inks which can be used in printing are highly viscous, filtering is difficult, defects are liable to occur due to dusts, impurities and gelled products of ink binders, and there are problems in accuracies of positioning and line breadth according to printing accuracy and surface smoothness.
A pigment dispersing method is a method of producing a color filter by a photolitho process using radiation-sensitive colored compositions comprising pigments dispersed in various photosensitive compositions. This method uses pigments, therefore, the method is stable against light and heat and as patterning is carried out by a photolitho process, positioning accuracy is sufficient and suitable for producing a color filter for a big image plane and a high definition color display.
When a color filter is produced by a pigment dispersing method, a film is formed by coating radiation-sensitive compositions on a glass base material by a spin coater or a roll coater and drying. The coated film is patternwise exposed and developed to obtain a colored pixel. This procedure is repeated every color and then a color filter is obtained.
A pigment dispersing method is disclosed in JP-A-1-102469 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-1-152499, JP-A-2-181704, JP-A-2-199403, JP-A-4-76062, JP-A-5-273411, JP-A-6-184482 and JP-A-7-140654, and negative type photosensitive compositions in which a photopolymerizable monomer and a photopolymerization initiator are used as an alkali-soluble resin are disclosed in the above references published.
However, in recent years, higher definition of liquid crystal display elements, further electric power saving of back light, and higher luminance are demanded, and further higher transmittance and higher contrast of a color filter are required.
From the manufacturing viewpoint of a color filter, higher sensitization of photosensitive compositions of a color filter, a broader development latitude, pigments having high dispersion stability which do not precipitate are demanded.
For attaining a high transmission factor of a color filter, there are methods of reducing the content of a pigment component in a photosensitive composition for a color filter or thinning a film thickness but the chroma of a color filter is reduced and a color filter becomes whitish as a whole due to these methods and color sharpness is lost.
Dianthraquinone-based pigments (e.g., PR-177, etc.) have so far been mainly used as a red pigment, as disclosed in JP-A-1-254918 and JP-A-2-153353. However, even these pigments cannot cope with the above-described demands in recent years, and a higher transmission factor cannot be obtained even by fine dispersion of these pigments.
As a novel method which does not require pigment dispersion, one method is disclosed in JP-A-8-6242, in which after an image is formed by molecularly dispersed pigment precursors, the pigment precursors are converted to pigments by a chemical process, a thermal process, or a photolysis process (a latent pigment). According to this method, since pigment conversion by a chemical process, a thermal process, or a photolysis process cannot be performed sufficiently, satisfactory results have not been obtained. In particular, as degrees of conversions of pigments differ according to temperature, pigments develop different colors according to heating temperature. Therefore, heat resistance is not sufficient and this method has not reached the stage of a practical use yet.
Any of the foregoing conventional techniques cannot sufficiently satisfy recent demands for a color filter.